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Scientists identified three genes that make it hard to combine the two varieties - and also highlighted a possible solution

Chinese scientists have identified a genetic barrier that makes it hard to crossbreed African and Asian rice varieties - a discovery that could lay the foundations for creating high-yielding hybrids.

While both Asian and African cultivated rice varieties have distinct strengths, crossing them often produces offspring with sterile pollen and low yields.

A research team has now identified three genes that act as a barrier to reproduction - and also a possible way round the problem.

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"This discovery provides an important theoretical foundation and valuable genetic resources for breaking the reproductive barrier between rice species and breeding new, ultra-high-yielding hybrid rice varieties," the official China Science Daily reported last Friday.

Such Asian-African crosses "could potentially increase yields by more than 10 per cent compared with existing Asian hybrid rice varieties", one of the authors of the study told the newspaper.

The research, conducted by a team led by Wan Jianmin at Nanjing Agricultural University's State Key Laboratory for Crop Genetics and Germplasm Enhancement and Utilisation, was published in the peer-reviewed journal Science on 28 August.

African and Asian rice are the only two species out of the 24 in the Oryza genus that have been domesticated to produce food for humans.

Asian rice offers high yields and excellent eating quality, making it a global staple.

Meanwhile, African rice is generally lower-yielding but has other valuable traits, including the ability to tolerate high temperatures, drought and poor soils, alongside a natural resistance to various pests and diseases.

Scientists hope crossing the two will result in new varieties that combine high productivity with greater resilience.

Following years of research, the Chinese study identified a genetic barrier to crossbreeding these species that lies within one rice chromosome.

When the two species are crossed, a genomic region known as RHS3 aborts around half the first-generation pollen grains created, while the others develop normally.

They identified three genes behind this process, which they named mao, dun and jia - or spear, shield and armour.

African cultivated rice has the complete spear-shield-armour system, but Asian cultivated rice has only the armour gene.

In hybrid offspring, the team found, pollen grains received different gene combinations. Roughly half inherited the complete three-gene system and developed normally, while the other half inherited only the armour, aborted and died.

He Xiaodong, a doctoral student at the university and co-first author of the paper, said that a protein encoded by the spear - known formally as ORF10 - damaged the mitochondria supplying energy to rice cells, killing the pollen.

However, protein from the shield (ORF16) binds to the spear, neutralising its toxicity, while the armour gene (ORF4) works alongside the shield to send the neutralised spear to a cellular "recycling centre".

Crucially, the research team identified one genetic resource, known as a germplasm, that is broadly compatible with both varieties and which effectively stops the spear's activities.

When crossed with both Asian and African rice, this is enough to largely overcome the drop in fertility.

However, the team acknowledged that their research only provided a theoretical foundation for creating new hybrids, adding that further work was needed to create suitable agricultural materials and address issues such as grain quality and pest resistance.

China already has some of the highest rice yields in the world - roughly 7.2 tonnes per hectare (6,400lbs per acre) - but is investing heavily in research aimed at boosting crop production as part of its efforts to improve food security.

Recent developments include a study in the journal Vita about a new hybrid rice variety that can be cloned with almost 99 per cent efficiency, allowing farmers to replant harvested seeds without losing the crop's high yield and other desirable traits.

Meanwhile, a hybrid wheat variety known as Jingmai 189 reportedly achieved nearly double the national average yield in a trial in the Taklamakan Desert.

Source : msn